The function of visceral organs like the gastrointestinal tract, the urinary tract and the blood vessels is to a large degree mechanical. The following introduction refers mainly to the gastrointestinal tract but the invention relates to similar applications in other hollow organs.
In the gastrointestinal tract, contents received from the stomach are propelled further down the intestine and mixed with secreted fluids to digest and absorb the food constituents. The biomechanical properties of the small intestine in vivo are largely unknown, despite the fact that the distensibility is important for normal function, and altered mechanical properties are associated with gastrointestinal (GI) diseases. Data in the literature pertaining to the mechanical aspects of GI function are concerned with the contraction patterns, the length-tension relationship in circular and longitudinal tissue strips in vitro, flow patterns, the compliance and the tension-strain relationship. The methods traditionally used for clinical or basic investigations of the small intestine are endoscopy, manometry and radiographic examinations. Although these methods provide important data on the motor function, little attention has been paid to biomechanical parameters such as wall tension and strain and the relation between biomechanical properties and sensation. During the past two decades, impedance planimetry was used in gastroenterology to determine wall tension and strain in animal experiments and human studies. Impedance planimetry provides a measure of balloon cross-sectional area and is therefore a better basis than volume measurements for determination of mechanical parameters such as tension and strain in cylindrical organs.
GI symptoms are often associated with disturbances in motility and sensory function in the GI tract. Several studies attempted to investigate these properties by means of balloon distension. {Schultz & Corkeron 1994 31/id} Unfortunately, the primary mechanism for symptoms elicited by GI distension remains unclear. It is well known that distension of the gastrointestinal tract elicits reflex-mediated inhibition and stimulation of motility via intrinsic or extrinsic neural circuits and induces visceral perception such as pain. Previous studies demonstrated that mechanoreceptors located in the intestinal wall play an important role in the stimulus-response function. It is, however, a common mistake to believe that mechanoreceptors are sensitive to variation in pressure or volume. A large variation in the peristaltic reflex and perception has been found in various studies, suggesting that pressure is not the direct stimulus. Instead, the receptors are stimulated by mechanical forces and deformations acting in the intestinal wall due to changes in the transmural pressure. Thus, the mechanical distension stimulus and the biomechanical tissue properties must be taken into account in studies of the sensory-motor function in the intestine.
It is well known that the passive elastic behaviour of biological tissues is exponential. The exponential behaviour protects the organs including the intestine against overdistension and damage at high luminal pressure loads and allows the intestine to distend easily to facilitate flow in the physiological pressure range. In arteries, it has been demonstrated. that collagen bears circumferential loads at high stress levels. Since gastrointestinal tissue is rich in collagen, it is likely that collagen is a major determinant of the curve shape. The passive elastic behaviour (tension-strain relation) of duodenum in vivo is exponential and hence can play a role in protecting tissue against high stress. At high loads the mechanical behaviour is contributed mainly by the passive tension curve, whereas at low stress levels, that is in the physiological range, the active tension curve also affects the tissue behaviour. Thus, the distensibility in vivo depends not only on the passive properties but also on the physiological state of smooth muscle.
Mechanical properties have been studied in vitro in muscle tissue strips from various organs. The strips are mounted in a small organ bath between hooks so the strip can be elongated in a controlled way and the resultant force measured. This has made possible studies of isometric and isotonic muscle length-tension diagrams in vitro. Usually the tissue has been studied when influenced by drugs such as muscle relaxants and muscle stimulants, in order to study active and passive tissue properties. The passive curve is normally described as exponential whereas the active curve is bell-shaped, i.e. with a maximum. The maximum active tension is presumably reached at a level of optimum overlap between the sliding filaments in the intestinal muscle cells. in vivo no such method exists. Manornetry is used to record the contraction patterns but it gives no information about the passive mechanical properties but only indirect data on the force of contraction. Balloon distension techniques, which record balloon pressure and balloon dimensions such as volume and cross-sectional area, can provide a mechanical stimulus to the wall. But in the way these techniques have been used, data on the smooth muscle force have been sparse and control of passive conditions have been insufficient.